The present invention relates to a static mixer and method for mixing fluids, especially viscous fluids. More particularly, the invention is directed to a method and apparatus for mixing viscous liquids, such as polymers, using stationary baffles providing sinuous, non-parallel spiraling flow paths to promote thorough and homogenous intermixing of the fluids.
Static mixers of various constructions have been known in the art for some time, and are also referred to as stationary baffle mixers or interfacial surface generators. Representative examples of prior art static mixers are illustrated and described in U.S. Pat. Nos. 3,190,618; 3,620,506; 3,643,927; 3,652,061; 3,923,288; 3,947,939 and U.S. Pat. No. Re. 28,072, for example.
As discussed in the above prior art patents, it has previously been found that the creation of turbulence which is typical of a normal mixing process is not desired, or effective, or really possible in the mixing art where either viscous material or materials of different viscosities, such as an epoxy resin and a low viscosity liquid catalyst, are mixed. Instead, the mixing approach taken by the prior art involves the use of baffles or stream deflecting elements of a specific configuration within a conduit or passageway. Frequently, such devices require machine or block molding, casting, or sheet fabrication to obtain the desired baffle configurations. The consequent cost and difficulty of manufacture result in a relatively expensive end product.
Moreover, many of the prior art mixers provide less than complete mixing with polymeric fluids. Thus, viscous fluids traveling down a tubular conduit are in laminar flow with discrete streamline velocity vectors. Several prior art mixers do not produce proper mixing in such cases because their configurations tend to preserve and extend the velocity streamlines and fail to provide adequate radial mixing, i.e., movement of fluid from the wall to the center and back again, which is needed to effectively intermix the cross section and eliminate concentration, temperature or mass gradients.
The incomplete mixing of prior art mixers is also a significant problem when attempting to mix a relatively low viscosity liquid catalyst into a high viscosity resin. In this case, the low viscosity catalyst tends to channel along the walls of the passageway or conduit and resists intermixing with the polymer which tends to remain in the interior.
A further problem exists in liquid-liquid co-current extraction applications where static mixers are commonly used in order to insure uniform dispersions across the cross section. In such applications, many of the prior art devices exhibit centrifugal vectors which tend to segregate fluids with different densities. In these cases the lighter fluid would be near the center and the heavier fluid near the wall, resulting in ineffective performance.
One static mixer of particular note is described and claimed in U.S. Pat. No. 3,743,250, issued July 3, 1973 to Fitzhugh et al. entitled "Fluid Blending Device To Impart Spiral Axial Flow With No Moving Parts". The blender described in that patent comprises two series of identical, longitudinal spaced apart flow-guides with identical circumferential orientations and with opposite inclinations in opposite sides of a tubular housing. Fluid streams to be blended are guided in parallel, spiral paths axially through a multiplicity of successive blending zones formed between adjacent longitudinally spaced flow-guides. Despite the effectiveness of the Fitzhugh et al. device, it has been found that the static mixer of the present invention provides even more efficient blending.